1. Field of the Invention
The present invention relates generally to an apparatus and a method for transmit antenna diversity in a wireless communication system, and more particularly to an apparatus and a method for minimizing a Peak-to-Average Power Ratio (PAPR) in an Orthogonal Frequency Division Multiplexing (OFDM) communication system using a multiple transmit antenna.
2. Description of the Related Art
Generally, the most fundamental issue in wireless communication lies in how efficiently and reliably data can be transmitted through a channel. The next generation multimedia mobile communication system, which has been actively researched in recent years, requires a high speed communication system capable of processing and transmitting various information such as images and wireless data, different than an initial communication system providing a voice-based service. Accordingly, it is necessary to improve system efficiency using a channel coding scheme proper for a system.
Different from wire channel environments, wireless channel environments in a mobile communication system are subject to loss of information due to an unavoidable error caused by various factors such as multi-path interference, shadowing, electric wave attenuation, time-varying noise, interference, and fading. Further, this information loss deteriorate performance of the mobile communication system because it may actually cause a serious distortion in transmitted signals. Accordingly, in order to reduce the information loss, it is necessary to improve the reliability of a system by means of various error control techniques based on characteristics of channels. From among these error control techniques, an error correcting code is commonly used.
In order to remove the instability of communication caused by fading, a diversity scheme is used. The diversity scheme may be classified into a time diversity scheme, a frequency diversity scheme, and an antenna diversity scheme, i.e., a space diversity scheme. The antenna diversity scheme is a scheme using multiple antennas, which may be classified into a receive antenna diversity scheme using a plurality of receive antennas for application, a transmit antenna diversity scheme using a plurality of transmit antennas for application, and a Multiple Input Multiple Output (MIMO) scheme using a plurality of receive antennas and a plurality of transmit antennas for application. The MIMO scheme is a kind of Space-Time Coding (STC) scheme. The STC scheme transmits signals encoded by a preset encoding scheme through a plurality of transmit antennas, thereby expanding a time-domain encoding scheme to a space domain and achieving reduced error rate.
FIG. 1 is a block diagram schematically illustrating a conventional transmitter in a mobile communication system using an SFBC scheme, i.e., an OFDM communication system using four transmit antennas. Referring to FIG. 1, the transmitter includes a precoder 100, an encoder 102, a plurality of OFDM modulators 104, 106, 108, and 110, and a plurality of transmit antennas 112, 114, 116, and 118.
The precoder 100 codes Nt input symbols, e.g., four symbols x1 to x4, so that a signal rotation is generated on a signal space, and outputs vectors r1 to r4 including the four coded symbols. More specifically, the precoder 100 codes the input symbols into a precoding matrix so as to generate a complex vector r.
The encoder 102 receives the output from the precoder 100, bundles the four symbols into two pairs to vectors ([r1, r2], [r3, r4]), wherein each pair includes two symbols or elements. Further, the encoder 102 encodes each vector by an Alamouti scheme and performs a frequency-space mapping for the encoded vectors. Herein, an encoding matrix based on the operation of the encoder 102 may be expressed as shown in Equation (1) below.
                    [                                                            r                1                                                                    -                                  r                  2                  *                                                                    0                                      0                                                                          r                2                                                                    r                1                *                                                    0                                      0                                                          0                                      0                                                      r                3                                                                    -                                  r                  4                  *                                                                                        0                                      0                                                      r                4                                                                    r                3                *                                                    ]                            (        1        )            
In the encoding matrix of Equation (1), the number of columns corresponds to the number of transmit antennas and the number of rows corresponds to the number of used sub-carriers. The encoder 102 generates the four antenna signals (or vectors) [r1, r2, 0, 0], [−r2*, r1*, 0, 0], [0, 0, r3, r4] and [0, 0, −r4*, r3*], and outputs the antenna signals to the corresponding OFDM modulators 104, 106, 108 and 110, respectively. For example, the [r1, r2, 0, 0] is output to the first OFDM modulator 104, the [−r2*, r1*, 0, 0] is output to the second OFDM modulator 106, the [0, 0, r3, r4] is output to the third OFDM modulator 108, and the [0, 0, −r4*, r3*] is output to the fourth OFDM modulator 110.
The first OFDM modulator 104 allocates the symbols [r1, r2, 0, 0] from the encoder 102 to four adjacent sub-carriers, performs an Inverse Fast Fourier Transform (IFFT), converts IFFTed signals into Radio Frequency (RF) signals, and transmits the RF signals through the first transmit antenna 112. The second OFDM modulator 106 allocates the symbols [−r2*, r1*, 0, 0] from the encoder 102 to four adjacent sub-carriers, performs the IFFT, converts IFFTed signals into RF signals, and transmits the RF signals through the second transmit antenna 114. The third OFDM modulator 108 allocates the symbols [0, 0, r3, r4] from the encoder 102 to four adjacent sub-carriers, performs the IFFT, converts IFFTed signals into RF signals, and transmits the RF signals through the third transmit antenna 116. The fourth OFDM modulator 110 allocates the symbols [0, 0, −r4*, r3*] from the encoder 102 to four adjacent sub-carriers, performs the IFFT, converts IFFTed signals into RF signals, and transmits the RF signals through the fourth transmit antenna 118.
The symbols transmitted through the first to the fourth transmit antenna 112, 114, 116 and 118 are shown on a time-frequency plane as illustrated in (a), (b), (c) and (d) of FIG. 1.
As illustrated in FIG. 1, the SFBC scheme according to the prior art is characterized in that it performs the frequency-space mapping for the precoded symbols by the Alamouti scheme and transmits the symbols for which the frequency-space mapping has been performed through the antennas during one time interval.
In the MIMO-OFDM communication system using the scheme as described above, a high PAPR may be caused by the afore-described multiple carrier modulation. That is, because data are transmitted using multiple carriers in the MIMO-OFDM scheme, the final OFDM signals have amplitude obtained by summing up amplitudes of each carrier. Accordingly, variation width of the amplitude is wide. Further, when the carriers have the same phases, the variation width has a very large value. Therefore, the prior art deviates from a linear operation range of a high power linear amplifier and signals having passed through the high power linear amplifier have distortion. The high power linear amplifier must operate in a nonlinear range in order to obtain maximum power. However, due to the distortion as described above, a back-off scheme is used, which lowers input power and operates the high power linear amplifier in a linear range.
The back-off scheme drops the operation point of the high power amplifier in order to reduce the signal distortion. However, as the value of the back-off increases, power consumption may also increase and thus the efficiency of an amplifier may deteriorate greatly. Therefore, signals having a high PAPR deteriorate the efficiency of a linear amplifier.
Further, in a nonlinear amplifier, an operation point belongs to a nonlinear range, such that nonlinear distortion may occur. Additionally, a problem including a mutual modulation among carriers and a spectrum emission, etc., may occur.